A modular multilevel converter (MMC) is a circuit system in which output terminals of cell converters having switching elements such as IGBT for which ON/OFF control can be performed and DC capacitors are connected in series to each other, thereby voltage equal to or greater than the withstand voltage of the switching elements can be output. This circuit system is expected to be applied to a DC power transmission system (HVDC), a static synchronous compensator (STATCOM), and the like.
For example, according to Patent Document 1, an MMC comprises a plurality of cell converters connected in series (cascade). Each cell converter includes a main circuit composed of a switching element and a DC capacitor, external terminals to be connected in cascade to other cell converters, bypass means connected between the respective ends of the external terminals, and a self-power supply which obtains energy from the DC capacitor of the main circuit and drives the bypass means and the like.
Through ON/OFF control of the switching element, the voltage between the external terminals can be controlled to be the voltage of the DC capacitor provided in the corresponding cell converter, or zero. The bypass means is a short-circuit switch of normally-ON type which is capable of short-circuiting output of the cell converter when this cell converter has failed, and by this short-circuit switch, the output of the cell converter that has failed is short-circuited by the short-circuit switch, thus it is possible to continue its operation as the system even if the cell converter has failed.
That is, in each cell converter 105, as the bypass means, a short-circuit switch 209 of normally-ON type is connected in parallel to the output terminals. A self-power supply 208 is connected in parallel to a DC capacitor 203, and drive power for the short-circuit switch 209 is supplied from the self-power supply 208.
When the cell converter 105 is stopped, the drive power is no longer supplied from the self-power supply 208, and thus the short-circuit switch 209 turns on by itself to short-circuit the output of the cell converter 105, thereby it is possible to continue its operation as the power conversion device even if one cell converter 105 has failed.